The flux used for soldering generally has efficacy to chemically remove any metal oxides from a solder alloy and a metallic surface of an object to be joined which is subject to the soldering and to allow any metallic elements to move from and/or to a boundary between them. Therefore, the soldering using the flux enables intermetallic compounds to be formed between the solder alloy and the metallic surface of the object to be joined, thereby obtaining any strong joining.
On the other hand, components of the flux include any components which cannot be decomposed or evaporated by heating during the soldering and they remain around the soldered parts as the flux residue.
Meanwhile, together with recent advancement of miniaturization of electronic components, an electrode to be soldered in the electronic component becomes also miniaturized. Therefore, an area which can be soldered by the solder alloy is limited so that it may be impossible to get sufficient joining strength by only the solder alloy.
Accordingly, a technology has been proposed in which the electronic component is securely bonded by covering a circumference of the part to be soldered with underfill or resin mold as parts-bonding means for strengthening the joining by the soldering.
Here, when the flux residue remains around the part to be soldered, the flux residue hinders bonding the part to be soldered to the resin, so that it is impossible to maintain the strength thereof. For this reason, it is required to clean the flux residue for covering the circumference of the part to be soldered with the resin. However, it takes time and/or costs to clean the flux residue.
Accordingly, a technology for avoiding the cleaning of the flux residue has been proposed (See, for example, Patent Document 1) in which flux contains any thermosetting resin, electronic components and a substrate which are objects to be joined are securely bonded by the flux residue in which the resin is hardened by heating during the soldering.